Drive control for roving frame

ABSTRACT

A control for winding yarn in textile machines to rotate the bobbin at a speed which is inversely proportional to the diameter of the yarn wound on the bobbin including: a pair of cone pulleys with a transmission axially slidable with respect thereto and guided through a carriage. The carriage is threadedly engageable with an axially displaceable threaded member which in turn is connected to a compensating means whereby rotation of the cones axially displaces the threaded member through the belt and carriage, and this displacement is amplified by the compensating means thereby effecting the output of the pulley transmission and the rotational speed of the bobbin.

United States Patent [72] inventor Jean Frederic Herubel 2,175,108 10/1939 Casablancas 57/96 Guebwiller, France 2,870,597 1/1959 Hill et al. 57/96 [21] Appl. No. 805,786 3,089,302 5/1963 Williamson,.1r... 57/96 [22] Filed Mar. 10,1969 3,134,217 5/1964 Williamson, Jr... 57/96X [45] Patented Mar. 23,1971 3,345,813 10/1967 Long 57/96 [73] Asslgnee f: Primary Examiner-John Petrakes [32] P y Mar. 1968 Attorney Waters, Roditl, Schwartz, & Nissen [33] France [31] 143379 54 DRIVE CONTROL FOR ROVING FRAME ABSTRACT: A control f0! winding yarn in textile machines l0 12 Claims 8 Drawing Figs rotate the bobbin at a speed which 15 inversely proportional to the diameter of the yarn wound on the bobbin including: a pair [52] US. Cl 57/96, f ne pulleys with a transmission axially slidable with 57/99 respect thereto and guided through a carriage. The carriage is [51] InLCI. D0lh 1/04 threadedly engageable with an axially displaceable threaded of Search 92, member i turn is connected to a ompensating means 102 whereby rotation of the cones axially displaces the threaded member through the belt and carriage, and this displacement [56] References Clted is amplified by the compensating means thereby effecting the UNITED STATES PATENTS output of the pulley transmission and the rotational speed of 2,003,362 6/1935 Hendrickson 57/96 the bobbin.

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DRIVECONTROLFOR ROVING FRAME.

and which carries a-screw nut'which meshes with a rotary threadmember lying parallelto the slide; I

The rovings which'areformedfrointextile fibers and which are'fedlby the flyer f'rameare woundonbobbinsin layers lying concentric with the axes'of the frames;

In addition'to the verticalreciprocating movement of the assembly" of winding spindles, these spindles must have a rotational speed whichis inversely proportional torthe diameter of the textile material being wound, This'is necessary because thetensioning of the roving between supply rollersand the windin'g-unit depends on the rotational speed of the-spindles. Ifthe tensionis too great irregularities will be formed'inrthe roving and the wound bobbin will have an excessively hard ap-' pearance. On the other hand'too low a ICHSiOHLWilll'fiSllll in loose or floating roving which. lead to. tangling at the en'-- trance to the rotary frictionipieces'of the flyers= It is therefore necessary toensure thattthe speed'vari'ation' shall correspond to a sequenceof stepsin the winding operation which is deter-- mined. by the particular characteristics of the-roving'ancb the nature of the fibers-of whichztheroving is composed.

In certain'flyer framesthevariation in thespeedof the spindles is effectedbya speedf regulator whose conepulleys are; of hyperbolic profile; .the belt b'eingarrangedkto'moveatlancom stant speed in. the directionof the: axis of the'conepulleys; however,.thesespeed regulators have certaindrawbacks;

Thus, the variations in the. speediof thespindlesdoes not'reliably correspondto'the prescribed'sequenceofiwindingisteps,.

and frequently. the tensioning of 'therovingtvariesbetween'ithe beginning and the end of the :windingof the yarn: It is'rimpossible to remedy, this defect because' of the: variation in theytenresults in a bulky mechanical assembly which is therefore not for textile-machineswhichobviates the' drawbacks referred toherei'nabove;

In accordance withthe invention a rotary-threaded member which serves to shift a carriage is also axially shiftable under the effect of a'compensating' control system.

The invention consists in a control for textile machines comprising-a drivingcone pulley, driven cone pulleys, the axes of the'pulley being arranged substantially parallel to one another, a transmission belt operatively connected between the pulleysfor' transmitting torque from the driving pulley to the driven pulley, the transmission belt being arranged for axiallysliding movementalong to the pulleys, a carriage guiding the'belt, axiallymoveable therewith, operatively connected to a member and axially displaceable in response to movements of. said memberin. a first direction; and compensating means responsive to the displacement'of the member for effecting a further displacement of the member in asecond dire'ctionand a furtheraxial movement of the carriage and belt'to thereby sion' occurs as a. function of theprofile' ofthecone pulleys which are permanently arranged? fixtures in* the machine. Furthermore, in orderthat theysmay give eff'e'ct'tathenecesswry .speed' variation, the profiles of the. cone pulleys arecnecessarily given a steep slope, which gives rise todifficultiesiinsthe movement of thebelt in slippingto-one side.

* In order to exercise. moreeffectiveicontrol over the speediof the spindles, certain machinesemploy cone pulleys;v of' rectilinearprofile which varies the speed of thespindles, .thisvariation being modified by a supplementarymovement ofthe belt running over. the conepulleyse This movement of'the.belt takes place, with greater orl'esser rapidity according tothe modification or correction'whichis tobe applied to'the speed variation of the spindles. The sequenceofsteps in the winding operationis thusfinally determinedhby the-profileof the-cone pulleys and also bythe natureof the,sequencerofmovements of the belt; Themovementof thebeltis generally effected .byta guide forkxwhich isfixed to the cable which runsraround a cam whose: profile corresponds to the: nature of. the speed. cor rectiomwhich is necessitated. Oneof' the drawbacks of-this type of controlis that thecam'requires a.length-of'cable which corresponds tothe effective lengthofv the pathof'contactrof' the belt on: the cone pulleysandisrelativelylarge.andlnecess sitatesthe use, of a.camofexcessivelylarge dimensions. The size of the cam togetherqwith thattof the: control cable-Pas sembly make. it impossible to accommodate the speed eontrol effectthe'output torque of the driving pulley.

By means of this speedcontrol it is possible forthe driven cone pulley to' accurately control the sequence of speed changes inthe spindles without. constructing the cone pulleys with steeply sloping surfaces. It is possible to provide a corrective action by use of a particular cone pulley profile, with a slightvariation in" the angle of slope thereof, to provide a cor-- rective action, with greater accuracy, by means of asystem'for controlling'theaxial movement of a threaded member".

In a preferred embodiment the compensation means includes a cam which is connected to the. threaded member through an' intermediary assembly including a worm gear I whichis carried by the threaded member and, secondly, a

toothed gear which meshes with the worm gear and is fixed for movement with a shaft which is-mounted for rotation instationary bearings. This form of construction represents an assembly which takes up very little space and enables, simply by replacing the cam by another'cam of suitable profile shape,

ingmeans for returningthe device to its initial position and whichcomprises: an auxiliary motor which is connected, firstly, to thedriving cone. pulley and, secondly, through the intermediary of a freewheel assembly, to the threaded member so that the said driving cone pulley and the: said threadedlmember are driven in a direction'opposite. to that corresponding to their nonnalworking direction. Means for controlling the=energization of the said auxiliary motor, and

for mechanically uncoupling the drive means, serving to drive the driving'cone pulley, while the control is returning to its starting'condition.

Whenthe control is intended to serve for' controlling. the

rotation of thespindles of a flyer frame equipped with a rocker unit which is-used for controlling the vertical, reciprocating movements of these spindles, then the means provided for controlling the-energization of the auxiliary electric motor will alsoeffect mechanical disengagement of thenormal means provided for'driving the saidrocker-unit.

In a preferred. form of construction this means serving to controltheenergization of the auxiliary motor comprise an electric circuit equipped with limit switches responsive to. the rotation of thecompensating cam.

The invention'will now be'described with reference to the accompanying drawings wherein:

FIG. 1 shows an elevational view of 'an embodiment of a speed control in the position assumed at the start of aworking cycle;

FIG. 2 shows, on a larger scale, a detail of FIG. 1;

FIG. 3 shows a cross section taken substantially along the line III-III of FIG. 2;

FIG. 4 shows an alternative embodiment of speed control shown in FIG. 2;

FIG. 5 shows the speed control of FIG. 1 in the position at the end of a working cycle and further provided with an assembly for returning the control to its initial position;

FIG. 6 shows a partial, side view, of the spaced control along the line Vl-VI of HO. 5, and

FIGS. 7 and 8 show a detail of the speed control shown in FIG. 6 in two different positions and on an enlarged scale.

The speed control shown in FIG. 1 includes a driving cone pulley 21 and a driven cone pulley 22 which are respectively carried by two parallel shafts 23, 24, and also a belt 25 which extends about the two cone pulleys and through a guide disposed in a carriage 27 which is movable along a slide 28 which lies parallel to the axes of the two cone pulleys. The carriage 27 carries a threaded portion 32 which threadedly engages a rotary threaded member 33 which extends parallel to the slide 28. The threaded member is disposed on a shaft 29 between two bearings 36 and 37 mounted in the machine frame 38.

The driving cone pulley 21 can be driven at different speeds by any suitable means (represented diagrammatically by a toothed gear 41 attached to a shaft 23 on the cone pulley 21 and an interchangeable gear 42 operatively engaging the gear wheel 41 In operation the driven cone pulley 22 rotates a vertical orientated rotatable spindle 45 belonging to a flyer frame.

Each spindle 45 on which yarn 47 is wound is rotatably mounted in a support 48 which is slidable along a column 49 in a vertically reciprocating manner under the control of a suitable drive means. The spindle 45 is slidable along a vertical orientated splined shaft 53 the lower portion of which carries a pulley 54 over which runs a belt 55 which passes over a further pulley 56 attached to the lower end of a vertically orientated intermediate shaft 57 the lower end of which is connected to the shaft 24 of the driven cone pulley 22 through bevel gears 58 and 59.

Flyers 62 are orientated so as to be coaxially rotatable with the spindle 45 by a belt transmission 63.

The threaded member 33 is rotatable about its axis, axially slidable. The threaded member 33 is rotated intermittently by a toothed gear 71 which meshes with a long-toothed pinion gear 72 secured to a shaft 73 which is mounted for rotation in the machine frame 38 and driven through a chain of other suitable transmission 74 by an assembly or rocker unit 76. The rocker unit is arranged to effect reverse reciprocating movement of the spindles. The rocker unit 76 also serves to reduce the magnitude of the alternating working strokes of the spindles and to vary the rotational speed of the spindles as a function of the increase in diameter of the textile (roving) 47 being wound on the bobbin. This latter function is carried out by the threaded member 33 which is driven by the train of motiontransmitting elements hereinabove described.

The axial sliding movement of the threaded member 33 is efiected by a compensating cam 81 which is operatively connected to a roller 82 which is carried at a central point of a lever 83 and which pivots, at one of its ends, about a pin 84 carried by the machine frame 38. A finger 85 (see also FIGS. 2 and 3) carried by lever 83 engages in a grooved ring 86 which is freely rotatably mounted on the respective end of the shaft 29 of the threaded member 33 and is retained on this shaft 29 by a disc 87 which is held in position against a shoulder on the shaft 29 by a nut 88 which cooperates with a lock nut 89.

The lever 83 is biased for pivoting about pin 84 associated therewith in the direction in which the roller 82 is urged against cam 81 by a spring 92. One end of the spring is attached at 93 to the machine frame 38 and the other end is attached to one end of a rod 95. The other end of the rod is hooked on to the free end of the pivotable lever 83.

Cam 81 is attached to a shaft 101 which is rotatable in bearings 102 carried by a support 103 which is secured to the machine frame 38 and which also carries the swivel pin 84 about which lever 83 pivots. Also attached to shaft 101 is a toothed gear 105 which meshes with a worm gear 106 rigidly secured to tee shaft 29.

In the embodiment shown in FIG. 3, cam 81 is readily interchangeable with another cam different configuration by means of a locating portion 107 to which it is connected by means of a nut 108 located at one end of the locating portion 107, which is threaded and engages an associated aperture in the cam 81.

The start and end of a working cycle are accurately determined by two limit switches 111 and 112 whose actuating rollers 113 and 114 are respectively pushed back by two fingers 115 and 116 which are secured to shaft 101 which carries the compensating cam.

The embodiment operates as follows:

At the start of a working cycle the control is in the position shown in FIG. 1. The belt 25 passes between the driving cone pulley 21 and the driven cone pulley 22 at the maximum and minimum effective diameter respectively. The cone pulleys are formed such as to minimize the slipping of the belt when the machine starts. This position corresponds to the maximum speed of the driven cone pulley 22 and to the start of the formation of a winding of the roving or yarn on the bobbin.

At the start of each fresh layer or group of layers of roving on the bobbin 47, the rocker unit 76 acting through the chain transmission 74, the toothed pinion gear 72 and the toothed gear 71 cause the threaded member 33 to rotate about its axis through a predetermined angular displacement. The rotation of the threaded member 33 results in a certain amount of displacement of the carriage 27 and the belt 25, in the direction of arrow f1, i.e. in the direction such as to reduce the diameter of the driving cone pulley 21 over which the belt passes and such as to increase the diameter of the driven cone pulley 22 over which the belt passes, so that there is a reduction in the speed of rotation of the spindle 45 which carries the package of roving or yarn being wound on the bobbin.

This angular displacement of the threaded member 33 results in an angular displacement of the same magnitude of the worm gear 106 and, consequently, a corresponding angular displacement of the toothed gear wheel 105 and of the shaft 101 which a carries the latter. The compensating cam 81 thus rotates through the angular displacement so that the roller 82, which is urged against the cam. causes the lever 83 to pivot the pin 84 in the direction of arrow f2 which causes the threaded member 33 to be displaced axially for a corresponding amount. The carriage 27 is thus shifted not only as a result of the rotation of the threaded member 33 but also under the effect of the axial displacement thereof. This, of course, has an accelerated compensating effect.

The profile shape of the compensating cam 81 is determined so that the axial displacement of the threaded member 33 is appropriate to ensure that the rotation of speed of the spindle 45 varies according to a predetermined required sequence. FIG. 2 shows the initial position 0 of the roller 82 about the cam 81 and 12 successive positions (numbered as 1- -l2). The first four positions corresponding to an effective portion of cam 81, while the last eight positions (5-12) correspond to a functionally noneffective portion of the cam, i.e. the portion of the cam which does not cause axial displacements of the threaded member 33. The corrective displacements of the threaded member 33 brought about by positions 1-4 of the roller along the cam in the course of a working cycle.

It is possible to use cone pulleys 21, 22 with linear generatrices and effect the entire compensation by cam 81 or, alternatively, to use cone pulleys of hyperbolic generatrices and bring about half of the necessary corrective action. In so doing there is no problem with excessive inclination of the belt on the cone pulleys for it is unnecessary in that half of the corrective action is performed by the cam. In the regions where the slope of the cone pulleys would, in fact, be too great, the effective parts of the cam which operate over the rest of the cone pulleys, in which the generatrix is relatively flat (of small concavity), the roller would be in: contact with the functionally inactive portion of the cam 81.

FIG. 4 illustrates a modification of the embodiment of FIGS. 1 and 2. Theessential difference is that theworm gear 1696A is stationary rather than being fixed to theshaft 29. The worm gear 106A-is fixed to the sleeve 121 whichrotates in bearings 122 carried by the machine frame 38. Shaft 29 is axially slidable in the sleeve 121 butbeing prevented from rotating with respect thereto by a key 123'which is secured in the worm gear 106A and is mounted for axial sliding motion in a longitudinal groove 124 in the shaft29. Thesleeve 121 is axially retained in thebearing 12 2 byashoulder' 127 and a ring 128, the latter of which is fixed to the sleeve by retaining pins or keys 129.

In this modification of the. invention, the same angular displacement of thethreaded member as in the caseof the embodiment of FIG. 2 results in the corresponding displacement of the carriage 127 which-serves to guide the belt. But this same angular displacement of the threaded member 33wliich results in the corresponding angular displacement of the cam 81. as in the previous embodiment causes a lesser axialdisplacement of the shaft'29 and therebysthe threaded member 33, because this axialsliding movement of the lead screw does not result in a slidingmotion of the worm gear, accompanied by a supplementary. rotation of the ,cam, such as to increase the extent'ofthe pivotal movement-oflever'83, (as in the case of the embodiment'ofFlG. 2). Y

In order to obtain the same axial displacements of the threaded member-33 it is necessary to give the effective part of the cam of the. embodiment illustrated in-FlG. 4 a steeper slope than is in the FIG. 2 embodiment.

FIG. 5 illustrates an apparatus which enables all the components of thespeed control to be automatically brought back tothe position which they assume. at the start of a cycle for winding yarn or roving on a bobbin.

This apparatus includesan auxiliary motor 151 which is connected, through a belt transmission 152, firstly tothe shaft of the drivingcone pulley 21' and, secondly, to the shaft 73 operatively connected to the threaded member 33.

The belt 152 passes over'a pulley 155 fixed on the shaft of motor 151, over'a pulley 156' attached to the shaft of the driving cone pulley 21, and overa pulley l57'which isfreely rotatably mounted on shaft 73 (for example with the interposition of roller bearings 161, 162) and forms part of a free wheel assembly generally designated as l63'(seeFIG. 7 also). The rotor 164 of this freewheel unit' is integral with shaft73 owing to the provision of a key 165. The wedging elements of the free wheelunit are formed by rollers 167 lying between the cams 168 of the rotor and the cylindrical bore 1690f the hub 171 of the pulley 157. The freewheel unit operates so that when the rotor 164 has stopped or rotates at 'a low speed during the working phase of the drive action of the threaded member 33, the pulley 157 can be driven at high speed in the direction of arrow fii by'the shaft ofcone pulley 21,. which rotates inthe direction of arrow f3 (FIG.- 6), and through the belt 152, and at the sametime drives the auxiliary motor 151 which is no longer energized but turns under no-load conditions.

Allthe other components of thev speed control are the same as those shown in FIG. land have been similar components to its initial position is as follows:

During'the active portion of the winding cycle for winding roving oryarn on the bobbins (FIG. 1), the cone pulleys of the speed control are driven from the'control means 42, and the threaded member 33 is driven from shaft'18l of the'rocker unit (as already explained above). The auxiliary motor 151 is When the active portion of the winding cycle has con- I cluded, the limit switch 112 (FIG. 3) is actuated by a finger 116' carried by the shaft 101 which is attached to the compensating cam 81 is attached. Means (not shown) will effect mechanical uncoupling of the driving means for controlling the driving cone pulley 21, mechanical uncoupling of the drive 180 of the shaft 181 of the rocker unit, and energization of the auxiliary motor 151, whose circuit connections are so arranged that the motor will now rotate in the opposite direction to that in which it was rotating when driven by the cone pulleys of the speed control inthe no-load condition, during an active portion of the workingcycle for winding roving or yarn. The auxiliary motor 151 which drives through the chain transmission 152 and the free wheel unit 163'in the direction of arrow f7 FIG. 8, the threaded member 33 in the direction opposite to that of its rotary movementduring the effective part of the winding cycle of movement; At this time the auxiliary motor 151 also drives, throughthe transmission 74 and shaft 181, the rocker unit 76 which is now mechanically uncoupled. The carriage 27 then returns the belt 25, in the opposite direction to arrow 11, along the cone pulleys which are also rotated by the auxiliary motor and by thebelt 152, while'the compensating cam 81 also turns towards its initial position. When the two elements have returned to their starting position shown in FIG. 1, the limit switch 111 is actuated by finger 115 which discontinues energization of the auxiliary motor and reengages the couplings of the drive 42, serving to drive the cone pulley 21, and of the drive means 180 serving to drive the rocker unit 76. The assembly is then ready for another .winding cycle.

I claim: 4 1

1. A control for textile machines comprising'a driving cone pulley, a driven cone pulley, the axes .of the pulleys being arranged substantially'parallel to one another,a transmission belt operatively connected between the pulleys for transmitting torquefrom the driving pulley to the driven pulley, the

transmission belt being arranged for axial sliding movement along to the pulleys, a member, a carriage for guiding. the belt axially movable therewith, operatively connected to the member and axially displaceable in response to movementsof the member in a first direction and compensating means responsive to the displacement of the member for effecting a further displacement of the member in a second direction and a further axial movement of the carriage and belt to thereby effect the output torque of the driving pulley.

2. A control as claimed in claim 1, wherein the said member is threaded and the said carriage hasa threaded portion and the operative connectionis a threaded engagement.

3; A control as claimed in claim 2, wherein the compensating means includes a cam means for' rotating the cam in response to. a displacement of the member, a pivotally mounted lever connected to the member and including a fol lower portion responsive to the rotation of the cam whereby the displacement-of the member in the first direction effects the rotation of the earn which pivots the lever which in turn further displaces the member in the first direction.

4. A control as claimed in claim 1 wherein means for returning the transmission belt to its initial axial position at the end of a working cycle are provided.

5 A control as claimed in claim 4 wherein the means for returning the'transmission belt includes means responsive to 1 the rotation of the cam to the end of the effective portion of the working cycle, said responsive means actuating an auxiliary driving means, said auxiliary driving means operatively connected to the driven pulley and the member for rotating the driven pulley and displacing the member in the reverse direction opposite to their normal directions of movement thereby returning the carriage-and belt to their initial position.

6. A control as claimed in claim 5, wherein means are provided for driving the driving pulley, and wherein the said responsive means actuate means to uncouple the means for driving the driving pulley.

7. A control as claimed in claim 6 wherein the auxiliary driving means is connected to the member through a freewheel assembly.

8. A control as claimed in claim 7, wherein a transmission belt is continuously connected between the auxiliary driving means, the driving pulley and a free wheel assembly.

9. A control as claimed in claim 6 wherein a bobbin is provided, transmission means connected between the driven pulley and the bobbin and means for winding thread on the bobbin whereby variations in torque and the rotational speed of the driven pulley are in effect responsive to the degree of winding of the material on the bobbin.

10. A control as claimed in claim 9, wherein a rocker unit is provided for reciprocating movement of the bobbin and varying the rotation speed of the bobbin in response to the increase in material wound on the bobbin, wherein uncoupling means for the rocker unit are provided and wherein this uncoupling means is also actuated by the said responsive means to uncouple the rocker unit.

11. A control as claimed in claim 5, wherein the said responsive means includes an electrical circuit including a limit switch.

12. A control as claimed in claim 1, wherein the pulleys are arranged so that the belt engages a relatively large diameter of one pulley as it engages a relatively small diameter of the other pulley and vice versa. 

1. A control for textile machines comprising a driving cone pulley, a driven cone pulley, the axes of the pulleys being arranged substantially parallel to one another, a transmission belt operatively connected between the pulleys for transmitting torque from the driving pulley to the driven pulley, the transmission belt being arranged for axial sliding movement along to the pulleys, a member, a carriage for guiding the belt axially movable therewith, operatively connected to the member and axially displaceable in response to movements of the member in a first direction and compensating means responsive to the displacement of the member for effecting a further displacement of the member in a second direction and a further axial movement of the carriage and belt to thereby effect the output torque of the driving pulley.
 2. A control as claimed in claim 1, wherein the said member is threaded and the said carriage has a threaded portion and the operative connection is a threaded engagement.
 3. A control as claimed in claim 2, wherein the compensating means includes a cam means for rotating the cam in response to a displacement of the member, a pivotally mounted lever connected to the member and including a follower portion responsive to the rotation of the cam whereby the displacement of the member in the first direction effects the rotation of the cam which pivots the lever which in turn further displaces the member in the first direction.
 4. A control as claimed in claim 1 wherein means for returning the transmission belt to its initial axial position at the end of a working cycle are provided.
 5. A control as claimed in claim 4 wherein the means for returning the transmission belt includes means responsive to the rotation of the cam to the end of the effective portion of the working cycle, said responsive means actuating an auxiliary driving means, said auxiliary driving means operatively connected to the driven pulley and the member for rotating the driven pulley and displacing the member in the reverse direction opposite to their normal directions of movement thereby returning the carriage and belt to their initial position.
 6. A control as claimed in claim 5, wherein means are provided for driving the driving pulley, and wherein the said responsive means actuate means to uncouple the means for driving the driving pulley.
 7. A control as claimed in claim 6 wherein the auxiliary driving means is connected to the member through a freewheel assembly.
 8. A control as claimed in claim 7, wherein a transmission belt is continuously connected between the auxiliary driving means, the driving pulley and a free wheel assembly.
 9. A control as claimed in claim 6 wherein a bobbin is provided, transmission means connected between the driven pulley and the bobbin and means for winding thread on the bobbin whereby variations in torque and the rotational speed of the driven pulley are in effect responsive to the degree of winding of the material on the bobbin.
 10. A control as claimed in claim 9, wherein a rocker unit is provided for reciprocating movement of the bobbin and varying the rotation speed of the bobbin in response to the increase in material wound on the bobbin, wherein uncoupling means for the rocker unit are provided and wherein this uncoupling means is also actuated by the said responsive means to uncouple the rocker unit.
 11. A control as claimed in claim 5, wherein The said responsive means includes an electrical circuit including a limit switch.
 12. A control as claimed in claim 1, wherein the pulleys are arranged so that the belt engages a relatively large diameter of one pulley as it engages a relatively small diameter of the other pulley and vice versa. 